1. Field of the Invention
Aspects of the present invention relate to an image forming apparatus using a fusing unit to fuse an image transferred onto a printable medium and, more particularly, to a fusing unit and an image forming apparatus using the same that is of an endless belt type.
2. Related Art
An electrophotographic image forming apparatus, such as a laser printer, a photocopier, a facsimile machine, or a multi-functional product, forms an electrostatic latent image by scanning light onto a photosensitive medium that is charged to a predetermined electric potential, develops the electrostatic latent image by a toner of a predetermined color, and prints an image by transferring and fusing the developed image onto a printable medium. A fusing unit is provided in a printing path of the image forming apparatus to fuse the transferred toner image onto the printable medium.
Fusing units are generally classified into two types: a roller type and a belt type. In the roller type, the toner image transferred onto the printable medium is fused by a heating roller and a pressing roller placed opposite each other with the printable medium in between. In the belt type, a contact surface between the printable medium and the heating roller is expanded using an endless belt wound on two pressing rollers.
The belt type fusing unit has the contact surface between the printable medium and the pressing roller. The contact surface is larger than a fusing nip of the roller type fusing unit. Therefore, there is no need to increase the external diameter of the pressing roller in order to expand the width of the fusing nip, which is formed by the pressure exerted between the heating roller and the pressing roller. The overall constitution can thus be made compact. Since the contact surface of the belt type fusing unit is comparably larger than that of the roller type fusing unit, the superior fusing performance of the belt type reduces warming-up time and fusing when printing at high speed.
FIG. 1 is a schematic sectional view of a conventional endless belt type fusing unit. FIG. 2 is a perspective view partially showing the fusing unit of FIG. 1. Referring to FIG. 1 and FIG. 2, the conventional fusing unit fuses the toner image 15 formed on the printable medium 10. The conventional fusing unit comprises a heating roller 3 having a heating lamp 1 therein, a first and a second pressing rollers 5 and 7 placed opposite to the heating roller 3 and elastically biased toward the heating roller 3, and a fusing pressing belt 9 wound on the first and the second pressing rollers 5 and 7.
Heat generated by the heating lamp 1 is conducted to the heating roller 3, heating a surface of the heating roller 3. The surface temperature of the heating roller 3 is maintained at a predetermined temperature. The first and the second pressing rollers 5 and 7 rotatably support the fusing pressing belt 9 and cause the fusing pressing belt 9 to come into partial contact with the heating roller 3 or the supplied printable medium 10. The first pressing roller 5 rotates and is elastically biased toward the heating roller 3 by an elastic member (not shown) to press the heating roller 3.
A width of the fusing pressing belt 9 is larger than a length of each of the first and the second pressing rollers 5 and 7 so that the heating roller 3 cannot come into direct contact with the first and the second pressing rollers 5 and 7. As shown in FIG. 2, an end part 9b of the fusing pressing belt 9 protrudes by a predetermined width in a lengthwise direction of the first and the second pressing rollers 5 and 7.
The hardness of the heating roller 3 is higher than that of the first pressing roller 5 so that a fusing nip N1 can form between the heating roller 3 and the first pressing roller 5. Accordingly, if the first pressing roller 5 having a comparatively lower hardness contacts the heating roller 3 at a part 5a, the part 5a is deformed toward the center of the first pressing roller 5 by the pressure between the heating roller 3 and the first pressing roller 5. The shape of the fusing pressing belt 9 is deformed at the part 5a where the fusing nip N1 of the first heating roller 5 is formed. As shown by a dotted line in FIG. 1, the fusing pressing belt 9, which is wound on the first and the second pressing rollers 5 and 7, becomes loose compared to a winding state of the fusing pressing belt 9 before the fusing nip N1 is formed. As a result, both end parts 9b of the fusing pressing belt 9 protrude in the lengthwise direction of the first and the second pressing rollers 5 and 7. Of the end parts 9b, the part 9c where the fusing nip N1 is located is wrinkled or folded, as shown in FIG. 2. Accordingly, the folded part 9c is damaged while continuously printing, thus reducing the lifetime of the fusing pressing belt.
If the fusing pressing belt 9 is employed as described in connection with FIG. 1 and FIG. 2, the fusing pressing belt 9 may slide in the lengthwise direction of the roller as the fusing pressing belt 9 rotates on the first and the second pressing rollers 5 and 7. The movement of the fusing pressing belt 9 in the width direction is called a belt bias. The belt bias may cause the fusing pressing belt 9 to come into contact with a guiding unit (not shown) provided at both sides of the fusing pressing belt 9 to guide the rotation of the fusing pressing belt 9 as well as to support the first and the second pressing rollers 5 and 7. The part of the fusing pressing belt 9 in contact with the guiding unit may be damaged by friction, thus reducing the lifetime of the fusing pressing belt 9.